For a Meatless Monday healthy lunch idea, try Falafels which are typically made from ground chickpeas and/or fava beans and served in a warm pita bread. Falafels are typically fried but you can also find them baked which will cut down the fat content.
The beans are a good source of non-animal protein, and the falafel in pita is a nice change of pace from your typical sandwich.
I enjoy eating falafels with some hummus, cucumbers and tomatoes.